(1) Field of the Invention
The present invention relates to an image reading apparatus in which image sensors are arranged in parallel to a main scanning direction and at separate positions in a sub-scanning direction to output color component values of each of picture elements of image data read from an original image.
(2) Description of the Related Art
An image reading apparatus in which R, G and B image sensors are arranged in parallel to a main scanning direction and at separate positions in a sub-scanning direction is known. The R, G and B image sensors read out image data from an original image and output color component values (RGB) of each of picture elements of the image data.
In the above image reading apparatus, the times the color component values (RGB) of each picture element are output by the image sensors are different due to the differences between the positions of the R, G and B image sensors in the sub-scanning direction. Therefore, the positions of the color component values of each picture element output by the image sensors may deviate from a desired position of the picture element, and a discrepancy of color in output image data may be produced.
To avoid this, in the above image reading apparatus, a correction of image data in which the difference between the positions of the R, G and B image sensors in the sub-scanning direction is taken into account is performed.
Suppose that an original image is optically read in the sub-scanning direction by the R, G and B image sensors in this order. Also, suppose that both a difference between the positions of the R image sensor and the G image sensor in the sub-scanning direction and a difference between the positions of the G image sensor and the B image sensor in the sub-scanning direction correspond to "N" lines in the original image.
For example, the above-mentioned correction of image data is performed such that, when the B image sensor reads out image data (B) from a present line, image data (G) read out from a corresponding preceding line by the G image sensor is delayed by the "N" lines, and image data (R) read out from a corresponding preceding line by the R image sensor is delayed by the "2N" lines. It is possible that the R, G and B image data of each picture element from the R, G and B image sensors be output at the same time.
However, the image reading apparatus generally has vibrations of the scanning mechanism such as a variation of the sub-scanning speed of the R, G and B image sensors. Due to the variation of the sub-scanning speed, positions of color component values of each picture element may deviate from a desired position of the picture element. Even if the correction of image data in which a difference between the positions of the image sensors in the sub-scanning direction is taken into account is performed, it is difficult to carry out a correction of the image data in which the variation of the sub-scanning speed of the R, G and B image sensors is taken into account. Since an image reading apparatus has vibrations of the scanning mechanism, the discrepancy of color in the output image data may be produced and the reproductivity of black may become low. It is impossible for the above image reading apparatus to prevent the positions of color component values of each picture element from deviating from the desired position of the picture element due to the variation of the sub-scanning speed.